This invention relates to a brushless DC motor and more particularly to improvements on a control system for brushless DC motor which is capable of obtaining rotor position detection signals without resort to rotor position detecting sensors but from motor terminal voltages by using filters and comparators.
A conventional brushless DC motor is equipped with a counter electromotive force voltage based rotor position detecting circuit (hereinafter simply referred to a position detecting circuit) comprised of filters and comparators and adapted to set up rotor position detection signals from motor terminal voltages. Such a motor is disclosed in, for example, JP-A No. 59-162793 filed by Hitachi, Ltd. in Japan on Dec. 26, 1975, Japanese Patent Pulbication No. 59-36519 filed by the same applicant in Japan on May 28, 1976, and "Brushless Motor without a Shaft Position Sensor", Trans. of the Institute of Electrical Engineers (IEE) of Japan, Vol. B. 105, No. 5, 1985.
As is referred to in the paper mentioned above, when the load upon the conventional brushless DC motor equipped with the position detecting circuit is increased, the phase of the rotor position detection signal, referenced to a motor induced voltage E.sub.o, leads in proportion to an increase in winding current due to an increased load. Accordingly the phase of the winding current, which is in phase with the rotor position detection signal, also leads, degrading motor efficiency and in an extremity that the leading angle measures about 30.degree., causing a failure to detect rotor positions, which leads to stoppage of the motor.
The above problems will be specifically explained below with reference to FIG. 1 illustrating a voltage vector diagram obtained when the brushless DC motor is controlled with rotor position detection signals from the position detecting circuit.
Referring to FIG. 1, given a motor current (motor winding current) I.sub.M, a voltage V applied to the motor is equal to a vector sum of motor induced voltage E.sub.o, voltage drop rI.sub.M due to a winding resistance and voltage drop XI.sub.M due to a winding reactance. The position detecting circuit prescribes in principle that the rotor position detection signal as indicated by PS in the vector diagram and the winding current I.sub.M be in phase with the applied voltage V.
Thus, as the winding current changes from I.sub.M to I.sub.M ' in accordance with an increase in load, a solid-line vector diagram shifts to a dotted-line vector diagram as shown in FIG. 1.
This means that commutation leading angle .gamma. indicative of a phase difference between motor induced voltage E.sub.o and winding current I.sub.M increases as the winding current increases.
Especially where the commutation leading angle .gamma. exceeds 30.degree. as mentioned previously, the winding current increases with an increase in commutation leading angle .gamma. even when the load stops increasing further, with the result that a positive feedback is established through increased commutation leading angle, increased winding current and increased commutation leading angle to increase the commutation leading angle .gamma. more and more and in an extremity, the motor output torque falls below the load torque, thus stopping the motor.
The reason why the phase of the rotor position detection signal produced from the position detecting circuit is shifted by the winding current is that the rotor position detection signal is in phase with the motor application voltage phase which changes with the winding current. Incidentally, since the position detection signal is obtained from the motor terminal voltage by using the filter and comparator, a change in the revolution number of the motor causes the phase angle of the filter to change according to its frequency characteristics. For example, when the revolution number of the motor increases, the frequency of motor terminal voltage is increased to increase the phase angle of the filter so that the commutation leading angle for motor control is changed for decreasing.